This invention relates generally to oscillators and, more particularly, to a temperature compensated triggerable oscillator whose phase upon triggering is predetermined and independent of its phase before triggering.
Oscillators are found in most instrumentation, from simple clocks to complex signal generators and oscilloscopes. Generally, an oscillator is utilized to produce a stable, periodic time-varying output waveform which serves as the information or timing signal for signal processing circuits. In a digital oscilloscope, for example, an oscillating signal is utilized for taking samples of an input signal. It is desirable, therefore, that the oscillating signal be as stable as possible and maintain a frequency and period that are constant as the environment of the instrument changes.
One condition that seriously affects the stability of an oscillator is temperature. As temperature of a transistor-based oscillator increases during the gradual warming of an instrument, the transistor parameters within the oscillator change and cause the oscillator frequency to drift. Depending on the type of oscillator, the nominal frequency may increase or decrease in the face of increasing temperature. Prior attempts to correct for temperature drift have focused on either (1) making currents within the oscillator proportional to base-emitter junction voltages or (2) modifying the basic circuit configuration to eliminate the dependence of frequency on base-emitter junction voltages. Neither approach has proven to be entirely satisfactory. The first approach usually provides only a partial compensation; the second approach results in increased circuit complexity and requires the use of pnp transistors, which limit the high frequency capability of the oscillator.
It is also desirable that an oscillator utilized for measurement start from the same phase each time the oscillator is triggered. For example, the trigger jitter around signal points to be repetitively sampled can be minimized if the oscillator always starts its measurement from the same phase. Prior oscillators suffer from the drawback that the oscillator phase upon triggering is often a function of the oscillator phase before triggering.